The company also said it will re-release a Security Bulletin from July 2008 (MS08-037) to add protections to the Domain Name Service (DNS) under Windows 2000 Service Pack 4.

Of the six December bulletins, three are designated "critical" and three are "important."

Jerry Bryant, senior security program manager lead at Microsoft, said in an e-mailed statement that Microsoft customers should make deploying the Internet Explorer Bulletin (MS09-072) a priority due to its "critical" rating, its Exploitability Index rating of 1, and the existence of proof-of-concept exploit code.

Other computer security experts concur.

"The highly critical vulnerability in IE6/7, with an exposure window to exploits of more than three weeks without the availability of a patch, should put the task of getting users off IE6/7 on the top of IT admins New Year's resolutions for 2010," said Wolfgang Kandek, CTO of Qualys, in an e-mailed statement. "They have to be migrated to a more modern browser, with the most viable options being IE8 with its well known patching mechanism or Firefox 3 with its more aggressive patching schedule."

Kandek observes that MS09-072 is the only bulletin this month that affects Windows 7 and Windows 2008 R2.

After the Internet Explorer fix, Rapid7 security researcher Josh Abraham says that MS09-071 is the second most critical vulnerability because it allows privilege elevation and remote code execution on Windows Vista and Server 2008.

"These exposures will be used in client-side wireless attacks and many enterprise customers are still not taking wireless security seriously enough," he said in an e-mail.